A number of human herpes viruses are significant pathogens which cause a variety of well known syndromes. They form a particular hazard to immunologically compromised patients such as those undergoing transplant surgery or suffering from AIDS. The Herpes simplex viruses, HSV-1 and -2, are the best known herpes viruses and cause recurrent facial and genital cold sores, respectively. In more severe cases, HSV-1 may cause herpes keratitis (leading to sight impairment) or a potentially fatal encephalitis. The HSV-1 genome has been sequenced and contains over 70 genes, approximately half of which have a role in virion structure and assembly. The HSV-1 virion (2,400 Angstrom units) consists of a large (1,250 Angstrom unit) icosahedral capsid embedded in an amorphous protein layer, called the tegument, which is in turn enveloped by a glycoprotein containing lipid membrane. Following infection of cells, herpes virions are uncoated at the cell membrane and the capsids are transported across the cytoplasm to the nuclear pore complex at which point, the viral DNA is released into the nucleus. New capsids (containing 7 proteins) are assembled inside the nucleus and, following packaging of viral DNA, fill capsids (containing 5 proteins) exit the nucleus by budding through the nuclear membrane. The long term objective of our research is to understand the structural basis of assembly mechanism for herpesvirus capsids and its interactions with other non-capsid proteins and cellular carriers/receptors. Such an understanding should prove valuable in developing interventionist strategies directed against herpesvirus infection. This proposals aimed at improving our knowledge of the herpesvirus capsid structure and molecular interactions using high resolution electron cryomicroscopy and computer reconstruction together with molecule biology techniques. The first phase of the proposed research is to demonstrate the feasibility of attaining a 10 Angstrom Unit structure of the herpesvirus capsid by employing our 400 kV electron cryomicroscopy coupled with improved image processing procedures. The reconstruction at 10 Angstrom resolution can be expected to define protein subdomains and reveal characteristic features, including the alpha helices under favorable circumstance, of individual capsid shell protein subunits. The second phase of the work envisaged in this proposal is to employ the developed procedures to study the molecular structures of naturally occurring, this proposal and genetically engineered herpesvirus capsids. The recent development of a baculovirus based system for HSV-1 capsid assembly allows extensive manipulation of the constituent capsid proteins and creates a great need of structural information on the newly generated capsids. Our structural studies will be aimed at answering biological questions relating to the structural transformation of the capsid proteins at various morphogenetic stages involving scaffold assembly, proteolysis, scaffold exit and DNA packaging; the molecular boundaries and interactions among the major capsid shell proteins and their respective contribution to the capsid stability; the mode of interactions of the capsid shell proteins with the tegument proteins; and the mapping of regions of interest of various proteins by inserting or deleting short fragments of amino acid sequences. We anticipate that this research will not only advance a useful tool or virus structure research but also shed critical insights into structure-based mechanism of biological activities of herpesvirus capsids.